Harriet Harman has been around for so long that she is now deputy to the office junior who used to chase after her retrieving her coat from wherever she had left it. That gives her a solidity at the despatch box. She ploughs on, unembarrassable. Which means that the main interest of Prime Minister’s Questions when the principals are away is usually how Nick Clegg handles the Conservatives behind him.

There was plenty of interest there today, but first Harman unexpectedly managed to embarrass herself. When she got to the fifth of her six questions, she decided to take Clegg up on his claim that the Liberal Democrats exercised real influence in the coalition Government. Yes they had, she said. Without them, we would not have had the bedroom tax, the tripling of tuition fees and a top-down reorganisation of the NHS. “Even I know the difference between the brake and the accelerator.” That not only sounded like everyday sexism but reminded everyone that she has been done for driving without due care and attention.

Also, it was pointless pantomime politics that ended with Clegg bellowing, “Without the Liberal Democrats there wouldn’t be a recovery.” Which, of course, the Tories loved and cheered loudly.

It was left to junior MPs of all three parties to ask more subtle questions that made life hard for Clegg and teased out a few telling answers.

Lucy Powell and Phil Wilson, the Labour MPs, asked sharp questions designed to divide Clegg from the Tories. Powell asked about the expected reintroduction tomorrow, in the Autumn Statement, of the Married Man’s Allowance, emphasising the middle word. The Deputy Prime Minister muttered in gender-neutral language about the “so-called marriage tax break”, but didn’t say anything.

Wilson asked him what he thought of the Tory party’s “hostility to the European Union”, which he said was “bad for business and bad for British jobs”. George Osborne, sitting next to Clegg, shook his head dismissively at this pathetic attempt to cause mischief. “I agree,” said the Deputy Prime Minister.

After this, time dragged a bit. The session lacked the edge, and volume, of the real thing, although Speaker John Bercow still managed to interrupt a few times to spoil what fun there was. I tried counting how many MPs were on Twitter on their phones or iPads, but there weren’t many, so I went back to wondering why Iain Duncan Smith, Work and Pensions Secretary, always stands at the end of the chamber opposite the Speaker’s Chair. Suddenly Bercow called out, “Charles Kennedy!”

His name wasn’t on the order paper, but the Speaker has to call occasional MPs who “catch his eye” to alternate questions from government and opposition sides. I hadn’t even noticed that the Lib Dem former leader was in the chamber.

Kennedy’s question was long, elaborate and a bit too clever. He asked Clegg to congratulate the Chancellor, Foreign Secretary and Prime Minister on their pro-EU actions rather than their Eurosceptic words. The Deputy Prime Minister suspected a trap and praised his predecessor’s mischievous wit instead.

The final awkward question was the last one, from Peter Bone, the Eurosceptic Tory who usually asks questions about what would happen if something horrible happened to the Prime Minister, to which the answer is, no, Clegg would not take over, but Clegg does not want to say so. This time, though, Bone started by saying, “I think he’s turning into a Tory,” and tested his hypothesis by asking if the Deputy Prime Minister supported the (illegal) extension of immigration controls on Romanians and Bulgarians beyond the end of the month. He didn’t, but he did repeat his support for the Government’s tough rules on claiming benefits, which many Lib Dems must find almost as aggravating.

The net effect of the session was to suggest that Clegg, Cameron and Osborne are the leaders of a centre party, besieged by Labour on the other side and Eurosceptic Conservatives on their own. A position that is probably more uncomfortable for the Prime Minister, in his absence, than for his deputy.